The present invention relates to the manufacture of clinker in industrial kilns, during which smaller amounts of CO2 are emitted, and which by its nature makes it possible to obtain a higher mechanical strength and in particular its compressive component in hydraulic cements.
Portland cement clinker is normally manufactured using an intimate pulverized mixture of limestone, clay, iron or other minor components; this mixture is exposed in a rotary kiln to temperatures of greater than 1450° C., ensuring the formation of the phases which are the source of the characteristics required for the cements. These phases are alite (C3S), belite (C2S), calcium aluminate (C3A), calcium aluminoferrite (C3A), calcium aluminoferrite (C4AF) and other minor phases. Alite (C3S) provides mainly the properties of strength, which normally retains values of greater than 50%, requiring in order to do this large amounts of limestone and consequently generating a high level of CO2 emissions.
It is important to remember that the CO2 emissions related to an industrial process for the production of clinker for Portland cement are due mainly to two phenomena: the decarbonation of the limestone, which changes from CaCO3 to CaO and then to CO2, and the combustion during the conversion of carbon of the latter materials into CO2, effectively combined with atmospheric O2. In the past, efforts have been undertaken to reduce the CO2 emissions by modifying the chemical and mineralogical composition of the clinker and that of the Portland cement; these modifications have made it possible to remove the alite phase from the clinker and to significantly increase the belite content with conversion into an alternative form of clinkers and belite cements.
It is known that, with belite clinker, cement having a low reactivity and consequently having a low initial compressive strength is obtained, although, at advanced ages of 28 days and more, the strength becomes higher, without it ever being comparable to that of the cements having a high alite (C3S) content.
This low compressive strength during aging makes it difficult to commercialize this type of cement since this characteristic does not meet the requirements over time of modern constructions imposed by the construction industry and does not meet the quality standards for cements.
An example of this technology for the production of belite cements is described in U.S. Pat. No. 5,509,962 of Apr. 23, 1996, in which Fluvio J. Tang describes the method for the production of a cement mainly composed of α-belite, the more reactive category of belite which is not generally encountered in cements produced under normal conditions. Tang shows in this patent how the use of Na, K and Fe ions makes it possible to activate and fix this category of belite, while attempting to improve the initial strength of the belite cements.
Another example is represented by British patent GB 2 013 648 of Aug. 15, 1979, entitled “Process for the manufacture of cement”, in which Richard Schrader explains how, by using the same process for the production of Portland cement and with an appropriate formulation of raw meal, which normally employs a smaller amount of limestone matter (CaCO3), a β-ferrite clinker is obtained at a temperature as low as 1350° C., the belite of which clinker is activated according to an appropriate cooling technique.
Sulfoaluminate-based cements are the fruit of efforts made by scientists and technologists to find an alternative to Portland cements. Cements of this type are composed mainly of the C4A3S (calcium sulfoaluminate) phase replacing the C3A and C3S phases in the clinker and the cement and reducing the emission of CO2. Generally, the proportion of C4A3S in clinkers of this type exceeds 40%, requiring starting materials rich in alumina and sulfates in order to be able to form said phase. Even if it is certain that materials having a high sulfate (SO4) content are commonplace, as in gypsum, anhydrite and other materials, alumina-rich starting materials are unusual and can be regarded as special, resulting in a high cost of supplying; bauxite is an example of this. In addition, deposits are not homogeneously distributed geographically.
Clinker and cement of this type use less CaCO3, due to its composition and to its special formulation, resulting in a significant decrease in the CO2 emissions, which can nevertheless reach 50% during the decarbonation process, as a function of the starting materials used and of their formulation.
Cement of this type has the distinguishing feature of exhibiting a high initial strength due to the formation of large amounts of ettringite (C6ASH32). However, as it is manufactured under special cost and production conditions, the employment thereof has been reserved for special uses, such as repairs and pavings.
Another characteristic of these cements is the temperature level at which they are manufactured. As this temperature is less than 1300° C., energy consumption is thus lower, therefore reducing the CO2 emissions.
Some examples of this manufacturing technology are disclosed in U.S. Pat. No. 6,149,724, of Nov. 21, 2000, and U.S. Pat. No. 7,150,786, of Dec. 19, 2006. In these documents, Ulibarri and Kunbargi present us respectively with their processes for the manufacture of cements having high proportions of calcium sulfoaluminate (C4A3S), with a low proportion of belite (C2S) and, for Ulibarri, at least 1% alite (C3S), in order to obtain a high initial strength during the reaction of the calcium sulfoaluminate C4A3S in order to form ettringite (C6ASH32), which also reduces the setting time. This cement is often used as a mixture with Portland cement in different proportions in order to improve its role.
In both the abovementioned cases, use is made of special starting materials, that is to say materials which do not require a plant for producing the Portland clinker or at least in amounts recommended by these inventors. Under these conditions, starting materials having very high alumina (Al2O3) contents are necessary, which materials are generally rare and of economic value, resulting in a very high cost for the production of cements of this type.
In recent years, another group of scientists has developed novel cements using technologies (belite and sulfoaluminate cements) described above in this document, in order to improve the role of belite cements, and also the perception on the market of the production of cements of this type having a slow change in the compressive strength.
Another example of this case is represented by United States patent application US 2007/0266903, of Nov. 22, 2007, in which Ellis Gartner shows us how a belite cement, prepared with a clinker manufactured at a temperature of between 1150 and 1350° C., composed mainly of α-belite devoid of alite (C3S) and the formulation of which uses a series of minor components, is improved by mixing it with calcium sulfoaluminate (C4A3S). In this case, this component is incorporated in order to activate the belite and to increase the initial strength, which is normally very low when only a belite cement is used.